Modern technology requires increasingly the ability to detect very small quantities, down to parts per billion, of a first gas and a second gas. The high sensitivity analyzers provided for this purpose have calibration problems that are difficult to solve, particularly for hygrometers. The known processes, of the type defined above, using a dilution of the first mixture with the second flow, permit covering at the utilization outlet only a limited range, the passage to another range requiring a modification of the means for injecting into the first flow the predetermined quantity of the first gas.